The present invention relates to a new and improved construction of a smoke detector having pulse-operated or intermittently operated radiation source.
Generally speaking, the smoke detector of the present development is of the type wherein a radiation receiver is arranged externally of the region directly irradiated by the radiation source. This radiation receiver, in the presence of smoke in the radiation region, is impinged by scattered radiation and delivers output pulses. Additionally, there is provided an evaluation circuit which contains switching elements which, when the output pulses exceed a predetermined threshold during a predetermined number of pulses, transmits a signal to a trigger or switching stage for delivering an alarm signal.
Such type of smoke detector is known to the art from Swiss Pat. No. 417,405 and the corresponding U.S. Pat. No. 3,316,410, granted Apr. 25, 1967. With such prior art smoke detector a radiation source is controlled by a pulse transmitter and transmits briefly lasting radiation pulses. The evaluation circuit connected with the scattered radiation receiver is controlled by the pulse transmitter of the radiation source in such a manner that, upon reception of scattered radiation only during the pulse phases of the radiation source, is it capable of delivering an output signal. Spurious pulses which arise between the radiation pulses are therefore blocked in the evaluation circuit and cannot lead to triggering of an alarm signal. What is disadvantageous with this equipment design is that spurious pulses which happen to occur during the same time as the radiation pulses can nonetheless trigger a faulty alarm signal.
To avoid this shortcoming it has already been proposed to connect an integrator or counter in circuit after such smoke detector which operates in coincidence. This has been described in detail, for instance, in Swiss Pat. No. 580,848 and the corresponding U.S. Pat. No. 3,946,241, granted Mar. 23, 1976. Notwithstanding these measures such type of smoke detector can still trigger false alarms in the presence of rapid successively occurring disturbances, such as those caused, for instance, by high-frequency electromagnetic radiation.
Furthermore, a scattered light smoke detector operating in coincidence is known from European patent application No. 14,779. Here, the evaluation circuit contains a counter device or counter which counts both the radiation source pulses and also the output pulses of the radiation receiver and whenever there prevails an uneven counter state following a random radiation pulse the counter is reset to null, however upon reaching a predetermined even counter state there is triggered a signal. However, also with this smoke detector there is not precluded the occurrence of spurious or false alarms, since in the presence of high-frequency electromagnetic disturbances during each pulse there can be generated a spurious pulse. Additionally, the circuit is complicated in design and therefore less reliable in operation.
A further problem existing with the previously described type of smoke detectors resides in the temperature-dependency of the radiation transmitter. In the case of optical smoke detectors, wherein there is used a projection lamp as the light source, the temperature-compensation can be accomplished through the use of a thermistor. Significant in this regard is the smoke detector disclosed in British Pat. No. 1,172,354, published Nov. 26, 1969.
With most of the employed semiconductor elements the transmitted radiation markedly decreases with increasing temperature. Attempts have been made to compensate such radiation decrease in that there is connected a NTC-resistor (negative temperature coefficient-resistor) in series with a light-emitting diode (LED) Motorola, European MOS Selection 1979, 9-334). However, the resistance values of the NTC-resistor tend to vary to such a great extent that the thus obtained compensation is not adequate.